Ultrahigh-frequency apparatus



March 4, 1952 A. F. BREWER 2,587,590

ULTRAHIGH-FREQUENCY APPARATUS Filed July 26, 1946 Patented Mar. 4, 1952.ULTRAHIGH-FREQUENCY APPARATUS Alexander F. Brewer, Lynbrook, N. Y.,assignor to The Sperry Corporatio Ware a corporation of Dela-Application July 26, 1946, Serial No. 686,304

(ci. 25o-6) 2 Claims. l

This invention relates to ultra-high-frequency apparatus and moreparticularly to ultra-highfrequency bridge circuits and theirapplications in ultra-high-frequency communication systems.

An object of this invention is to provide an improved microwavecommunication system in which a single oscillator serves as atransmitter and as the local oscillator for the microwavesuperheterodyne receiver.

A further object of this invention is to provide an improved microwavecommunication system in which the transmitter and receiver are connectedto a single antenna in such a manner that the transmitted power does notharm .the receiver which is in a continuously operative condition.

Another object of this invention is to provide an improved communicationsystem having a single antenna in which the receiver is in an operativecondition at all times, thus eliminating the necessity of a push-to-talkswitching system.

Still -another object of the present invention is to provide an improvedmicrowave communication system using two antennas and utilizing themaximum amount of power available in the system.

Another object of the present invention is to but two parts. each ofwhich travels down the tween the two balance arms, the input andoutprovide an improved microwave diversity-type communication system.

Briey, the present invention is characterized by the use of a microwavebridge adjusted to opcrate in an inconventional manner. Microwavebridges are known which consist of four adjust-3` able elements or armsarranged in a symmetrical fashion, similar in function to the lowfrequency bridges. One such bridge using wave guide arms has been termeda magic Tee and is described below. In the present system, one such armor.

element of this type of bridgeis designated as the input arm and isadapted to receive electromagnetic energy which distributes itself amongthe other arms of the bridge in amounts dependent upon the adjustment ofthe remaining arms. The arm which is symmetrically opposite the inputarm is for convenience called the output arm, while the remaining armssymmetrically positioned between the input and output arms aredesignated as the balancing arms. The fundamental property of suchbridge circuits is that the energy which enters the bridge through theinput arm separates at the junction of the arms into three parts, eachof.which is transmitted down one of the remaining arms. However, undercert-ain conditions of impedance balance, it is possible to cause theinput energy to split into put arms of the bridge may be interchangedwithout changing the operation of the'system, i. e., the ratio of outputenergy to input energy remains constant. i

When such a bridge circuit is used in a microwave system, the variousarms may be shifted functionally so that the arm which is designated asthe input arm for one function may become a balancing arm when thebridge is used in a different manner. The arm through which energy isbeing supplied to the bridge during any particular function of thesystem is called, at that time, the input arm, the two adjacent armsbeing designated as balancing arms, and the arm which is symmetricallyopposite the input arm is then called the output arm.

By using a microwave bridge in an unbalanced condition, such asdescribed above, it is possible to construct a microwave communicationsystem in which a single oscillator serves as the microwave transmitterand at the same time provides a local oscillator wave for thesuperheterodyne receiver. In such a system the oscillator is-connectedto the input arm of the bridge. The antenna is connected to one of thebalancing arms, the second balancing arm being terminated preferably inits characteristic impedance, and the mixer of the superheterodynereceiver is connected to the output arm. In operation, the frequency ofthe oscillator is adjusted so as to differ from the frequency of thereceived signal by an amount equal to the intermediate frequency of thereceiver. Any impedance unbalance that exists between the secondbalancing arm (preferably terminated in its characteristic impedance)and the -antenna arm causes a fraction of the oscillator power which isfed into the input arm to appear at the output arm to which the mixerfor the receiver is attached. By proper adjustment, this fraction iskept at a sufficiently small value to prevent overloading the mixer.

The received signal which enters the system r through the antennadivides at the junction point of the microwave bridge so that a portionof it appears in the mixer arm. Since the frequency of the receivedsignal differs from the 'frequency of the local oscillator by an amountequal to the intermediate frequency of the receiver, the mixer isexcited by these two ultra-high-frequency signals and produces aresultant intermediate frequency output which is amplied and detected bythe remainder of the receiver.

The invention in another of its aspects relates to novel features of theinstrumentalities described herein for achieving the principal objectsof the invention and to novel principles employed in thoseinstrumentalities, whether or not these features and principles are usedfor the said principal objects or in the said field.

A further object of the invention is to provide improved apparatus andinstrumentalities -embodying novel features and principles, adapted foruse in realizing the above objects and also adapted for use in otherfields.

For a better understanding of the invention, in this and otherembodiments, reference is had to the following description taken inconnection with the accompanying drawings in which:

Fig. 1 is a perspective view of a wave guide type ofultra-high-frequency magic Tee bridge useful in explaining the presentinvention;

Fig. 2 is a general schematic representation of an ultra-high-frequencybridge circuit utilizing the bridge shown in Fig. 1;

Fig. 3 is a simplified schematic circuit diagram of an embodiment of thepresent invention used in an ultra-high-frequency communication systemhaving but one antenna; and

Fig. 4 is a simplified schematic circuit diagram of a further embodimentof the present invention used in an ultra-high-frequency communicationsystem using two antennas.

Referring now more particularly to Fig. 1, there is Shown a perspectiveview of a wave guide type of ultra-high-frequency bridge having fourarms I0, II, I2 and I3. According to conventional notation, arm I0 issaid-to be in series with arms I I and I2 and is said to form a seriesor .Eplane Tee. Arm I3 is said torbe in shunt with arms I I and I2 andis said to form a shunt or I-I-plane Tee. That is, arm Il] is coupled tothe electric field in arms II and I2 and arm I3 is coupled to themagnetic field in arms II and I2.

In order to better explain the theory of operation of the presentinvention as well as to discuss the operation of such a wave guide typeof bridge shown in Fig. 1, reference is made to Fig. 2 which is ageneral schematic representation of an ultra-high-frequency bridgecircuit. In Fig. 2 load impedances Z1, Z2, Z3 and Z4 are coupled to thearms of the wave guide Tee shown in Fig. 1. Arm- IIJ is considered to bein series with arms II and I2, whereas arm I3 is considered to be inshunt with arms II and I2. If arm It] is considered as the input arm,that is, if an input voltage is made to appear across impedance Z1, theinput electromagnetic wave will travel down theinput arm I0 and willdivide at junction point I 5 in a manner which is dependent upon therelative magnitudes of the impedances Z2 and Z3 which terminate thebalancing arms Il and I2 adjacent to the input arm I0. If these twoimpedances Z2 and Z3 are equal when viewed from the junction point I5,the input electromagnetic wave will divide into twovequal portions, oneportion travelling down arm II and the remaining portion travelling downarm I2. There will be no part of the input wave introduced into arm I0which travels down output arm I3, to output circuit element Z4.

However, a slight unbalance in the value of impedances Z2 and Z3 (astransformed to junction point I5) will cause shunt output arm I3 to beexcited, and a fraction of the energy introduced into input arm I willthen travel along output arm I3. The magnitude of this fraction will bedetermined by the degree of impedance unbalance between Z2 and Z3.

If the functions of arms Iii and I3 are reversed the same result will beobserved. In this case the electromagnetic energy which enters shunt armI3 divides at the junction point I5 of the bridge* If the impedances Z2and Za are equal when viewed from the junction point I5, the inputelectromagnetic wave will divide into two equal portions, one portiontravelling down arm II and the remaining portion travelling down arm I2..As before, an unbalance of impedances Z2 and Z3 will cause a fractionof the energy introduced into shunt arm if: to enter series arm i0, themagnitude of the fraction being depend s ent upon the amount ofimpedance unbalance.

The electromagnetic waves which travel down the arms II and I2 whenseries arm IIJ is the input arm are exactly opposite in phase at pointsequi-distanceirom the plane of symmetry of the arms II and I2. On theother hand, if electromagnetic energy is introduced through shunt armI3, the arms I I and I2 are excited by respective waves which arecoincident in phase at points equi-distance from the plane of symmetryof the arms I I and I2.

Either or both arms ii and IZ may be used as input arms as well as armsI and I3. Howe ever, if arm II or arm I2 is to be used as the input arm,both the series arm IQ and shunt arm I3 should be matched to the bridgecircuit so that when looking down either arms I I of: I2 theircharacteristic impedance Z0 is seen. I-f arm II is used as the inputarm, the energy which travels down it will divide, in the same manner asdescribed above, into two or three portions depending upon the impedancerelation of the two adjacent arms. Similarly, energy which is introducedthrough arm I2 will divide in a manner dependent upon the impedancebalance between the two adjacent arms.

It should be noted at this point that the pres-l ent invention is in noway limited to ultra-high-I frequency bridges composed entirely oi waveguides. Instead, the invention is completely' general and is adaptableto ultra-high-frequency bridges composed completely of coaxial elements,or combinations of coaxial and wave guide elements, or of other types ofelements such as the conventional two-wire transmission line elementsused at lower frequencies.

From the above discussion it is seen that, by adjusting the relationshipof the terminating impedances of the two opposite arms of the bridgecircuit, it is possible to cause a predetermined fraction of the inputenergy to appeary at the symmetrically opposite output arm.. Thisfraction can be'made zero merely by makingA the terminating impedancesof the balancing,`

arms equal to each other, so that the bridgey circuit is said to bebalanced. The results obtained by operating such a bridge circuit in aslightly unbalanced condition are very useful in manyultra-highFfrequency applications.

Fig. 3 is a simplified schematic circuit diagram of anultra-high-frequency communication system having two stations I and II,each employing a bridge circuit used in an unbalanced State as'above-described. Station I has a transmitter I connected to the inputarm lil of a bridge I. The mixer of a superheterodyne receiver l isconnected to the output arm I3 of bridge I. An antenna I is connected toarm I2 of bridge I, and its arm II is terminated in its characteristicimpedance Zo by a suitab-le circuit element 2| having that impedancevalue. The input impedance ofi-antenna I is adjusted to be slightlydifferent from the characteristic impedance of arm I2 to which it isconnected. By so doing, a slight impedance unbalance is obtained betweenarms II and I2 when viewed from junction point I5, so that a smallfraction of the energy which is introduced into the bridge I fromtransmitter arm I Il will be introduced into mixer arm I3, as discussedabove. Care must be taken to make the impedance imbalance sufficientlysmall so that the amount of energy which enters mixer arm I3 does notdamage the mixer of receiver I which is connected to this arm. Theimpedances of arms I and I3, when viewed from junction point I5, areusually kept equal so that the energy which enters arm I 2 from antennaI during reception will divide into but two portions, one portionentering transmitter arm III and the remaining portion entering mixerarm I3, with none of the energy being introduced into terminated arm II.

A similar arrangement is provided at station II, with a transmitter IIfeeding energy into bridge II through its input arm I D', the mixer of asuperheterodyne receiver II being connected to the bridge output anmI3', antenna II being connected to arm I2', and arm II' being terminateiin its characteristic impedance Z'o by circuit element 2|'. The sameimpedance relations are established at station II as at sta-tion vI,with a slight impedance unbalance between arms II' and I2', andimpedance balance between IIl and I 3. The operating frequency oftransmitter I is selected to differ from the operating frequency oftransmitter II by an amount equal to the common intermediate frequencyof the two microwave superheterodyne receivers I and II.

In operation, transmitter I is modulated in any conventional manner bythe intelligence it is desired to transmit from station I to station II.The energy from transmitter I travels along the input arm Ill of bridgeI and divides at the junction point I5. Thatportion ofthe modulatedenergy from transmitter I which enters antenna arm I2 will be radiatedby antenna I. The modulated energy or signal thus radiated by .antenna Iis picked up by antenna II and is fed into antenna arm I 2' of bridgeII. This energy divides at junction point I5' with the useful portionentering arm I3'.

In the meantime, transmitter II is oscillating at a frequency whichdiffers from that of transmitter I by an amount equal to the commonintermediate frequency of the two microwave receivers.`

This carrier frequency energy from transmitter II will enter bridge IIthrough transmitter arm I0 and will separate at junction point I5 with asmall fraction of it entering mixer arm I3' along with the usefulportion of the modulated or signal energy picked up by antenna II.

The carrier frequency wave from transmitter II which enters mixer arm I3adds to that portion of the modulated wave from transmitter I picked upby antenna II reaching mixer arm K miner I.

I3', and these waves are mixed by the mixer of receiver II to produce anintermediate frequency wave. The resultant modulated intermediatefrequency energy is vamplified and detected by the other conventionalstages ,of receiver II.

Communication in a reverse direction, from station II to station I, isaccomplished simply by modulating transmitter II. The modulated energyfrom transmitter II which enters the bridge through transmitter arm IIJdivides at a junction point I5. That portion of the input modulatedenergy from transmitter II which enters antenna arm I 2' is radiated byantenna II. This modulated signal is picked up by antenna I and entersbridge I through its antenna arm I2. The picked up signal energyseparates at junction point I5-of bridge I, with the uful portion of itentering mixer arm I3.

The carrier energy of transmitter I (which is, of course, unmodulatedduring a reception) enters the bridge I through transmitter arm III anddivides at junction point I5 in the same manner as before. That smallfraction of the carrier energy from transmitter I which enters mixer armI3 is mixed by the mixer of receiver I with the modulated signal energyfrom transmitter Il' which enters mixer arm I3, and the resultantmodulated intermediate frequency signal which is produced is amplifiedand detected by receiver I.

Thus, it is seen that, by the use of a bridge circuit in a slightlyunbalanced state caused by the intentional slight mismatch between theterminate-:i arm I I (or II') and the antenna arm I2 (or I2), acommunication system has been devised using a single antenna fortransmission as well as reception. Furthermore, by using a bridgecircuit in such an unbalanced condition, a small fraction of thetransmitter carrier wave is used to mix with the received wave therebyeliminating the need of a local oscillator for the superheterodynemicrowave receiver.

Another desirable feature of the communication system just described isthat, during transmission, the receiver of the transmitting station isin a fully operative condition, so that the speaker is able to hear hisown voice in his own receiver if he so desires. fact that duringtransmission part of the modulated radio frequency of the transmitterwill enter the receiver at the transmitting station. At the same timethe transmitter at the receiving station will provide an unmodulatedcarrier which, because of the frequency separation of the respectivecarriers, will act as a local oscillator for the re ceiving station.More specifically, referring to Fig. 3, let us assume that station I istransmitting a modulated carrier which emanates from trans This energypasses down arm Ii) and divides at the junction I5 into three parts, twoof which travel down arms I I and I2 to the terminating impedance andantenna, respectively. However, some energy, depending upon theimpedance unbalance existing between the balancing arms I I and l2. isintroduced into receiver I .by way of arm I3. At the same time stationII, which arranged to receive energy from station I, has its transmitterII in an operative condition. As a result, this carrier frequency whichemanates from antenna 2, is incident to antenna I and passes along lineI2 to the junction point I5 and divides solely between arms I0 and I3 asa func tion of the respective impedance balanceof these two arms.Inasmuch as the frequency separation ofthe two carrier Waves is equal tothe desired intermediate frequency, the two energies, which This followsfrom the' are introduced into receiver l, are successfully mixed.Accordingly, station I, although it is set. for transmission to stationII, is capable of hearing its signal in its own receiver I. I

`From the discussion of the method of operation of this system, it isseen that this is an indication that both the transmitter at thereceiving station (which is then being used as a local oscillator) andthe distant transmitter are in operation. If the speaker fails to hearhis own voice in the receiver, he immediately knows that there is abreakdown in the system. If the intelligence to be transmitted is otherthan voice, the receiver at the transmitting station may still be usedas a monitor by giving either an aural or visual indication.

From ii'ie above discussion it is seen that a material portion of theultra-high-frequency energy which is supplied to the input arm II] ofbridge I is introduced into terminated arm I I where it is absorbed bythe terminating impedance Zo. Also it is seen that only half the energywhich is picked up by antenna I is utilized in mixer arm I3, with thehalf of theareceived energy which enters transmitter arm III beingwasted. To overcome these losses and still provide many of theadvantages of the communication system described above, a slightlydiierent embodiment of the present invention is used as shown in Fig. 4,which is a general schematic representation oi an ultra-high-frequencycommunication system employing a bridge circuit used in an unbalancedcondition in combination with two antennas. Referring to Fig. 4, stationI comprises a transmitter I which is here connected to the balancing orside arm II of bridge I; receiver I which is connected to the oppositeside arm I2; and antennas A and B connected to series arm I!) and shuntarm I3 respectively.

The input impedances of antennas A and B are adjusted to be slightlydiierent when viewed from bridge junction point I5. Because of thisslight impedance unbalance, a small fraction of the input energy fromtransmitter I will enter receiver arm I2. On the other hand, theimpedances seen from junction point I5, when looking down arms I I andI2, are selected or adjusted to be balanced, so that energy which enterseither series arm II) or shunt arm I3 will divide into but two equalportions, With not energy entering the opposite arm.

Station II is arranged in the same manner as station I, with atransmitter II connected to side arm II' of bridge II, a receiver IIconnected to side arm I2; and antennas A and B' connected to series andshunt arms IU' and I3 respectively. The same impedance relations areestablished at station II as at station I, with arms I I and I2' beingbalanced and arms IB and I3' having a slight impedance unbalanced. As inthe previous embodiment, superheterodyne receivers I and II have acommon intermediate frequency, and the carrier frequencies oftransmitters I and II differ in frequency by an amount equal to thisintermediate frequency.

In operation, if it is desired to transmit signals from station I tostation II, transmitter I is modulated in any conventional manner by theintelligence which it is desired to transmit. The modulated signal fromtransmitter I enters bridge I through side arm I I and divides at thejunction point I5, one portion of the input energy travelling alongseries arm I and a second portion travelling along shunt arm I3, with avery small fraction being introduced into receiver arm I2.

enter arms I and I3 are radiated by antennas.

A and B respectively. At station II, antennas A and B pick up themodulated energy from station I. The modulated energy which is picked upby antenna A' lenters series arm Ill and divides at the junction pointI5', with a portion of it entering receiver arm I2 and a portion of itentering transmitter arm II. Since arm I0 is a series arm, the portionsof energy which enter arms I2 and II are 180 out' of phase at pointsequi-distance from junction point I5'.

In the same manner, the energy which is picked up by antenna B' is fedthrough shunt arm I3' and divides equally at junction point I5', with aportion travelling along receiver arm I2 and another portion travellingalong transmitter arm I I. In this case the phases of the two portionsat points equi-distance from junction point I5' are the same, since theinput energy is introduced through shunt arm I3.

It can be seen, therefore, that, by adjusting the relative phases of theenergy which travels down series arm I0 and shunt arm I3', it ispossible, because of the phase relations described above, to havereenforcement or cancellation of energy in the side arms II and I2'.Since it is desired to feed the energy picked up by antennas A and B' tothe mixer of receiver II, While it is desirable that no energy go totransmitter II, a phase adjustment is made so that cancellation resultsin transmitter arm II' and reenforcement results in receiver arm I2'.Such an adjustment could be made by shifting the physical location ofone of the antennas A and B or by placing a conventional phase shifterin either arm I0 or I3. Such phase shifters are indicated schematicallyat 22 and 22', connected to antennas A and A respectively. Since thefrequency of that portion of the unmodulated energy from transmitter IIwhich reaches the mixer of receiver II, differs from the frequency ofthemodulated energy received from transmitter I, modulated energy of thedifference or intermediate frequency is produced by the mixer ofreceiver II, which is then amplified and detected by the other stages ofreceiver II.

through arm I9 to transmitter I Where it is useless. In the system shownin Fig. 4, practically all of the transmitter power is radiated, withonly a small portion being utilized at the transmitting station as alocal oscillator for the receiver. Also, all of the energy which ispicked up by the two antennas is fed to the mixer of the receiver, and

substantially none is fed into the transmitter arm to be theredissipated. For these reasons, in those cases where it is desired tomaintain communication with the minimum amount of transmitter energy,this double-antenna system is more advantageous than the single-antennasystem.

As in the previous embodiment, the speaker is able to hear his own voicein the background. as he modulates his transmitter. Again this is an' 9indication to him that the system is operating correctly. Failure tohear his voice in the background indicates to him that there is abreakdown in the communication link.

An added advantage of the embodiment shown in Fig. 4 is that the use oftwo antennas at both stations results in a diversity type system. Insuch a diversity system, vthe two antennas have directionalcharacteristics but are oriented in slightly diierent directions so thatsignals are received, if possible, which travel over different spacepaths. The advantage of such a diversity system is that atmosphericeffects of a local nature which may interfere with one signal path mayhave no eiect on the second signal path, resulting in more reliablecommunication.

This system has all the advantages of that of Fig. 3, in that thetransmitter is used as the, local oscillator for a superheterodynereceiver. The receiver is in operative condition at all times and noadditional attenuation is required to keep the relatively hightransmitter power from damaging the input mixer of the receiver.

While the aforementioned embodiments serve to point out the advantagesof the present invention, they are merely illustrative of the broad useto which a bridge circuit in an unbalanced state may be put. Theinvention in its broad aspects may be applied in other systems and it isnot desired to limit the use of the invention only to the describedcommunication systems.

Since many changes could .be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In combination, a transmitter and a superheterodyne receiver whichoperate at frequencies which differ by an amount equal to theintermediate frequency of said receiver, a Wave guide interconnectingthe output of said transmitter and the input of said receiver, saidtransmitter providing the local oscillator for said receiver, first andsecond antennas, first and second wave transmission branchesinterconnecting said rst and second antennas respectively to theelectric and magnetic planes of said wave guide at a common junction,the impedances at said common A junction of the portion of said waveguide connected to said transmitter and the portion of said wave guideconnected to said receiver being substantially equal and the impedancesat said common junction of said iirst and second wave transmissionbranches being slightly unequal, and means for causing the Wave-energyreceived by said antennas to diier in phase.

2. In combination, a bridge circuit having an input element and anoutput element and two balancing elements all interconnected through acommon junction, separate antennas terminating each of said twobalancing elements, the impedances of said balancing elements beingunequal at said common junction, a transmitter having its outputconnected to said input element, and a superheterodyne receiver havingits input ccnnected to said output element, the operating frequency ofsaid transmitter differing from the operating frequency of said receiverby an amount equal to the intermediate frequency of said superheterodynereceiver and the transmitter providing the local oscillator for saidreceiver.

ALEXANDER. F. BREWER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date Re. 21,955 Chaifee Nov. 25, 19412,333,719A Herold Nov. 9, 1943 2,401,717 Wolff et al June 4, 19462,401,751 Friis June 11, 1946 2,408,791 Magnuski Oct. 8, 1946 2,412,935Tashjian Dec. 17, 1946 2,416,790 Barrow Mar. 4, 1947 2,424,156 EspleyJuly 15, 1947 2,425,314 Hansell Aug. 12, 1947 2,436,828 Ring Mar. 2,1948 2,445,895 Tyrell 'July 27, 1948 2,445,896 Tyrell July 27, 19482,447,392 Byrne Aug. 17, 1948 2,475,127 Carlson July 5, 1949 2,475,474Bruck et al July 5, 1949 FOREIGN PATENTS Number Country Date 466,014Great Britain May 20, 1937 551,472 Great Britain Feb. 24, 1943

